The present invention relates to the field of vehicles with multiple wheels, each of which are driven individually by a respective hydraulic motor. More particularly, this invention is a low cost electronic traction control system for such vehicles.
Various two-wheel, three-wheel, and four-wheel hydraulic drive arrangements are known for on-road and off-road mobile vehicles or machines. Two problems commonly associated with multiple wheel hydraulic drive arrangements are: 1) wheel slipping (overspeeding), and 2) wheel dragging (underspeeding). Wheel slipping or overspeeding results when the hydraulic motor drives its wheel at too high of a speed relative to the other wheels or the existing traction conditions. The wheel then tends to spin, making it ineffective for traction and steering purposes. Furthermore, the spinning wheel can consume all of the available workflow from the hydraulic pump, thereby starving the other motors and causing the vehicle to stop. Wheel dragging or underspeeding occurs when the wheel is stopped or slowed in an abrupt manner. Abrupt deceleration of the wheel can lead to turf damage, jerkiness, loss of steering control and undesirable tire wear.
One traditional approach that addresses these problems is to provide the circuit with hydraulic combiner/divider (C/D) valves. The C/D valve has a relatively low cost, but its inclusion in the circuit limits performance. The C/D valve works on the basis of a pressure differential across the supply and work ports. When a machine most needs positive traction, the flow from the drive pump is usually quite low. Therefore, the pressure drop across the C/D valve is usually not sufficient to properly engage the working spools of the valve. Without further operator intervention, a slipping wheel would consume all of the available workflow and the machine would stop. The operator normally must command more flow from the pump, which equates to greater traveling speed, in order to reach a threshold of flow that generates the necessary pressure drop to allow the C/D valve to perform its intended function of forcefully dividing flow to the various working motors. When working near a construction site, around people, ordinary equipment, an operator command to increase the work flow from the drive pump in order to engage the traction control function is not only undesired, but may also be unsafe. Conventional C/D valves also introduce considerable heat into a hydraulic circuit by virtue of the differential pressure drop. The pressure drop is proportional to the system flow and is therefore greatest when the machine is in a high speed traveling mode rather than during turning or low-speed off-road operation. Traction control is typically not required when the machine is traveling at higher speeds. Thus, heat is constantly being added to the system by the C/D valve, independently of the actual vehicle need for traction control. Heat is a natural enemy of a hydraulic transmission. Thus, there is a need for a non-complex, low cost electronic traction control system that offers improved performance.
Another approach that addresses these problems is taking a variable displacement pump and connecting it in closed circuit to a plurality of two-port, two-position, bi-directional, normally open, proportional flow control valves that control flow to a plurality of motors that rotate drive wheels. Though this improves upon previous methods, problems with this method still remain. Specifically, in order to accurately control the variable displacement pump, motor speed sensors need to be connected to the output shaft that connects each motor to each driving wheel in order to determine the rotational speed of the shaft. Then the sensors send an electronic signal to an electronic controller that controls the variable displacement pump. Implementing a plurality of speed sensors on each output shaft of each drive wheel is expensive. Furthermore, the transmission must have a closed circuit that causes pressure drops and inefficiencies known to closed circuits.
Therefore, a primary objective of the present invention is the provision of an improved electronic traction control system for a four wheel drive vehicle. Another objective of this invention is the provision of an electronic control system that is durable, economical to produce, and reliable to use.
These and other objectives will be apparent from the drawings, as well as from the description and claims that follow.